The invention relates in general to thermal regeneration apparatus for high temperature oxidation of pollutants in exhaust gas flows of industrial systems. More particularly, it relates to an improved regenerative thermal oxidizer that is more compact and more modular in construction than commercially available regenerative apparatus of comparable capacity by virtue of its having two cold-face areas per regenerative bed, which are in flow communication via an inlet/outlet crossover duct.
Purification of exhaust gas flow streams by regenerative thermal oxidization has been known for some time. Typically, regenerative oxidization apparatus includes at least two regenerative chambers containing heat exchange elements and a burner for heating the gas and oxidizing pollutants contained therein. In such an apparatus, gas to be purified is conducted to one of the regenerative chambers, which preheats the gas by virtue of a previous heat exchange step. From this inlet or gas heating regenerator, the gas flows to a high temperature combustion chamber containing one or more burners for oxidizing the pollutants in the gas. The gas is conducted from the combustion chamber to an outlet or cooling regenerative chamber, which cools the gas as heat from the gas is transferred to its heat exchange elements. The purified and cooled gas then is led to an exhaust stack for venting to atmosphere. Following a predetermined time cycle, the flow of gas through the regenerative system then is reversed. The outlet cooling gas regenerator now becomes the inlet heating regenerator and the previous inlet regenerator now functions as the outlet regenerator, which cools the gas prior to exhaust. The heat transferred to the outlet regenerator is recaptured by its stoneware, and used to preheat the inlet gas during the next cycle.
Regenerative oxidization apparatus may have only two regenerative chambers, such as disclosed in U.S. Pat. Nos. 4,671,346 to Masters et al. and 5,024,817 to Mattison. However, three-chamber designs, such as those disclosed in U.S. Pat. Nos. 3,895,918 to Mueller and 5,026,277 to York, deceased, are commonly employed to alleviate the problem of unburned gases in the inlet regenerator being released upon a reversal of flow cycles. As exemplified by these patents, regenerative oxidizer systems may incorporate at least three regenerative chambers with the odd chamber being in a dead or idle mode in which there is no flow to or from this chamber. During the dead mode, the gas present in the inlet regenerator is purged to prevent the release of untreated gases to atmosphere.
The last two mentioned patents also represent the two basic types of regenerative oxidizers in commercial use today--the horizontal flow type and the vertical flow type. In the horizontal flow type oxidizer, gas flows horizontally through the regenerative chambers, as is apparent from FIG. 11, which is a reproduction of FIG. 1 of U.S. Pat. No. 4,779,548 to Mueller et al. FIG. 11 shows a number of regenerative chambers 12' arranged radially about and in flow communication with a central high-temperature combustion chamber 11'. Each regenerative chamber comprises a bed of heat exchange elements confined by a radially inner retaining wall 13' at the hot-face area of the bed and a radially outer retaining wall 14' at the cold-face area of the bed. Loading of and access to the stoneware is provided by doors 15' located at the top of the chambers. Although not readily apparent from FIG. 11, FIG. 2 of the '548 patent and FIG. 2 of U.S. Pat. No. 3,895,918 show the regenerative chambers having cross-sectional flow areas that are tapered inwardly in a direction from the inner hot-face retaining wall to the outer cold-face retaining wall. During operation, gas to be purified is conducted into an inlet duct ring 24', which distributes the gas to the inlet regenerative chamber, i.e., the chamber having its inlet valve 30' in the open position. The gas then flows past the inlet valve into a vertical duct 19' adjacent the cold-face retaining wall 14' and flows horizontally through the regenerative chamber and the inner hot-face retaining member 13' into the central combustion chamber 11' where it is purified by high-temperature oxidation. The gas is then pulled through the outlet regenerative chamber, which cools the purified gas, and an exhaust duct ring 27' via an open outlet valve 30' of the outlet chamber. Before the next cycle of operation begins when the outlet regenerative chamber functions as the inlet regenerative chamber and vice-versa, the valves of the inlet regenerative chamber are closed and any residual gases in that chamber are flushed through the combustion chamber. This prevents residual unpurified gases from being drawn directly into the exhaust duct ring when the valves in the inlet chamber are reversed at the start of the next cycle.
The horizontal flow oxidizers of the above design work well and achieve high heat recovery efficiencies, typically 80-95%, due primarily to two reasons. First, the tapered design of the regenerative beds relieve pressure by providing an increasing cross-sectional area as the gas is heated when it flows from the cold-face to the hot-face area of the bed and a decreasing cross-sectional area as the gas is cooled when it flows in the opposite direction. Secondly, the flushing volume necessary to purge the regenerative bed is the smallest of any comparatively sized, commercially available oxidization apparatus to date, which improves destruction efficiency. However, despite these advantages, certain drawbacks in the horizontal flow design exist.
For example, for the industrial applications to which the invention is directed, a regenerative oxidizer typically must have a capacity capable of processing 2,000-25,000 s.c.f.m. (standard cubic feet per minute) of effluent at a heat recovery of 95%. For these capacities, the height of a horizontal flow oxidizer generally ranges from 10 feet to 20 feet while the width will be 25 feet. The height and width of these oxidizers results in considerable disadvantages and additional costs. First of all, these oxidizers must be shipped to the end user in a multitude of pieces and assembled on site because of the height and width limitations of standard truck deliveries. The general shipping size limitations are 13 feet-6 inches for the height when the unit is loaded on the truck and 12 feet for the width without the use of special, expensive escorts and 14 feet when the expensive escorts are employed. With these constraints, the combustion chamber, regenerative chambers, and the inlet and outlet manifold ducts of a typical industrial oxidizer must be shipped separately and the unit assembled on site. In addition, the height of the oxidizers necessitates the use of extensive platforming for access to components, such as the flow control valves and actuators, instruments, and stoneware loading doors (see the location of upper valves 30' and stoneware loading doors 15' in FIGS. 11and 12). The cost of the platforming is significant because it may constitute as much as 10% of the total cost of an industrial oxidizer, which typically may be $1 million or more. A more cost effective way to build and deliver an industrial regenerative oxidizer is to assemble as much of the oxidizer as possible in the factory and to minimize or eliminate the platforming. However, to date there is no commercially available regenerative oxidizer that is compact enough to be shipped by truck in modular regenerative units and can achieve the high heat recovery efficiencies of up to 95% in the given industrial capacities.
Another disadvantage of the horizontal flow oxidizer is the requirement for two retaining wall members, one at the hot-face area and another at the cold-face area of each regenerative chamber. These members are particularly susceptible to wear due to the high temperatures that must be endured and the exposure to corrosive gas that may occur in certain applications.
The flushing arrangement of the horizontal flow oxidizer also has certain drawbacks. As shown in FIG. 11, the minimum flushing volume that must be purged during flushing cycles consists of the vertical area 19' extending between the inlet and outlet valves 30', 30'. Using a typical industrial capacity of 10,000 s.c.f.m., the valves may be 2 feet in diameter and the length of the vertical duct 19' may be 10 feet long. This produces a volume of approximately 37 cubic feet, which, as demonstrated below, is less than the vertical flow oxidizers but still a considerable volume that must be flushed before each flow reversal. In addition, to ensure that the entire volume of unpurified gases in the bed 12' and duct 19' is flushed, a separate baffle member is usually provided to distribute the flushing air along the vertical extent of the cold-face retaining wall 14'. A typical baffle member comprises a perforated tube communicating with the flushing air, such as illustrated at 81' in FIG. 12, which shows a cross-sectional view through a regenerative chamber of a typical horizontal flow-type oxidizer.
The vertical flow type regenerative oxidizers generally are not as efficient as the aforementioned horizontal flow type and have certain other drawbacks. Examples of the vertical flow oxidizers are disclosed in U.S. Pat. Nos. 3,634,026 to Kuechler et al., 4,650,414 to Grenfell, 4,793,974 to Hebrank, and 5,026,277 to York, deceased. As illustrated in FIG. 13, which is a reproduction of FIG. 1 of U.S. Pat. No. 5,026,277, the vertical flow type of regenerative oxidizers comprise cylindrical cans 1", 2", 3" connected to a common combustion chamber 41" disposed thereabove. Each vertical can contains heat exchange material supported by a cold-face retaining member 4" disposed above a large enclosed space 5" having a diameter equal to the diameter of the can. During operation, gas to be treated flows through an inlet duct 19" via an open inlet valve 10" and space 5" where it flows vertically upward through the inlet or heating regenerative can 1" and into the combustion chamber 41". The gas then flows across chamber 41", vertically downward through the outlet or cooling regenerative can 3" and into the larger enclosed space 5", from where it flows to an exhaust duct 27" via an open outlet valve. Before flow reversal occurs, the inlet regenerative can is purged by connecting it to a source of negative pressure, which causes gas to flow through this can in a direction away from the combustion chamber.
For vertical can type oxidizers having industrial capacities of 2,000-25,000 s.c.f.m. with 95% thermal efficiency recovery, the height of the oxidizer typically will range from 15-20 feet, while the width or diameter of each can typically ranges from 8-30 feet. Thus, the same truck shipment limitations discussed above apply equally to the vertical can oxidizers. The cans containing the regenerative heat exchange material, the manifolds, valves, and purification chamber all must be shipped separately and assembled on site. The vertical can design also suffers from the same disadvantage requiring the use of platforming to access the valves and other components mounted at the top portions of the oxidizer.
Another significant drawback of the vertical flow type oxidizers lies in their reduced efficiency compared to the horizontal flow type because the minimum flushing volume is much larger than that of a horizontal flow device of comparable capacity. Again using a typical industrial capacity of 10,000 s.c.f.m., the diameter of a vertical can of an oxidizer of the type disclosed in York typically would be about 7 feet, while the height of enclosed space 5" under the can would be about 2 feet. This produces a minimum flushing volume of about 80 cubic feet, which is much more than the 37 cubic feet flushing volume of the 10,000 s.c.f.m. horizontal flow type oxidizer discussed above.
In addition, the vertical can oxidizers do not eliminate both hot-face and cold-face retaining members; at least a cold-face retaining member is needed, such as shown at 4" in FIG. 13. The York oxidizer also employs additional components such as a second blower and attendant valves, conduits, etc. not required in the horizontal flow oxidizers because York uses negative pressure to purge the inlet regenerator rather than positive pressure.
In U.S. Pat. No. 3,634,026 to Kuechler et al., an embodiment is proposed in FIG. 4 that which apparently does not require the use of hot-face and cold-face retaining members. In this proposal, two regenerative flue chambers 61'" are defined by dividing walls 62'" and central wall 62a'". The flues are in communication with a combustion chamber 64'" and a communication duct 66'" and or 67'", each of which must be provided with separate inlet and outlet ducts (and valves). To Applicant's knowledge, such a design was never marketed and appears to be a commercial impossibility. The slumping sides of heat-exchange material in ducts 66'" and 67'", as well as the large flow opening in ducts 66'" and 67'", the marrow flow opening between the bottom of dividing wall 62'" , and the bottom wall 72'", and the large flow areas of flues 61'", would produce extreme variations in air flow path lengths. Air flowing adjacent wall 62'" clearly has a much shorter flow path through the bed 63'" than air flowing adjacent wall 72'", which would result in intolerable heat exchange efficiencies. In addition, even if interlocking saddles were used as the heat-exchange material, the bed would not prevent the material from shifting due to vibrations occurring during shipping and/or during operation because of thermal contraction/expansion.
The foregoing demonstrates that there is a need for, and the invention is directed to the problems of providing, a regenerative thermal oxidizer having a capacity of 2,000-25,000 s.c.f.m. and a heat recovery efficiency rate of up to 95% that is shorter in height and more compact than commercially available oxidizers and has a modular regenerative unit construction that can be assembled in the factory and shipped via standard truck deliveries in one piece.